1. Field of the Invention
This invention relates to a thermal flow meter for detecting a flow rate of fluid such as air and the like and a flow rate measuring device utilizing the thermal flow meter.
2. Background Art
The thermal air flow meter using a thermo-sensitive resistor such as a heat generating resistor and a temperature compensating resistor and the like having a temperature characteristic can directly detect a mass amount of air, so that this air flow meter is widely used for measuring a flow rate of intake air for an internal combustion engine of an automobile and the like. A detected air flow rate signal is utilized for calculating a fuel injection amount of an electronic controlled fuel injection device and the like.
The thermal sensitive type resistor such as a heat generating resistor or the like of a type in which a platinum wire is wound around a bobbin and coated with glass has been widely used and in recent years, a thin film type in which a thin film resistor is formed on either a ceramic substrate or on a silicon substrate and a semiconductor type such as a polysilicon have been proposed.
As a flow rate detecting system, there have been provided one system in which a heat generating resistor mounted at a flow path is heated and controlled in such a way that a temperature difference between it and the temperature compensating resistor may become a predetermined difference and an electrical current flowing at the heat generating resistor is directly detected; and the other system in which temperature detecting resistors are arranged at both sides of the heat generating resistor and a flow rate is detected in reference to the temperature difference of the temperature detecting resistors and the like. Both of them basically utilize a resistive temperature characteristic which is changed when the thermo-sensitive resistor heat exchanges with fluid.
When a flow rate detecting element showing a relative high response delay in output is used, it is necessary to correct a response delay by inversely changing a time constant in delay in response. Such a correction as above for delay in response is carried out at the sensor side in a step (a pre-processing) performed before inputting a flow rate detecting signal to a control device (an engine control unit, for example) (the gazette of Japanese Unexamined Patent Publication No. 62012/1996 and the gazette of Japanese Unexamined Patent Publication No. 14418/1999 and the like) and carried out at the engine control unit (the gazette of Japanese Unexamined Patent Publication No. 10752/1994 and the gazette of Japanese Unexamined Patent Publication No. 13789/2003, for example).
The former is one example of a correction of characteristic of the flow meter (a sensor) performed through a digital correction, and the latter is one example of a system used at the engine control unit for improving a measurement error caused by a response delay of the sensor. Both of them are used for improving the delay in response when a sensor showing a high delay in response is utilized.
In addition, the thermo-sensitive type flow rate sensor has a non-linear output characteristic and also shows a pulsation caused by a blow-by gas at the engine and the like. Since they are causes for producing factors for error of output signal, the gazette of Japanese Unexamined Patent Publication No. 94620/1999 and the like have a description about one example in which the output signal is digitally corrected at the sensor circuit side in a digital manner and outputted to the engine control unit.
In the gazette of Japanese Unexamined Patent Publication No. 821/1987 is described a technology in which when there occurs a pulsation flow having a high pulsation amplitude of an amount of intake air and accompanied with a partial inverse flow at a region of low number of rotation of an engine with 4 cylinders or less or at a region showing a heavy load operation, the pulsation is to be corrected because an accuracy is decreased in the prior art air flow rate device.
In addition, as a method for decreasing an error in a flow rate output signal accompanied with a pulsation, the gazette of Japanese Unexamined Patent Publication No. 316145/1999 and the gazette of Japanese Unexamined Patent Publication No. 337382/1999 and the like have a proposal that an error correction is carried out as follows by using either the engine control unit or the controlling microcomputer. That is, a non-linear sensor output signal is converted into a linear signal (a flow rate) under application of a map to convert it into an air flow rate and the like, the pulsation amplitude is reduced by a filter, then it is changed into a non-linear line again to cause the amplitude of the final output signal to be reduced and the error is reduced.
[Patent Document 1]
Gazette of Japanese Unexamined Patent Publication No. 62012/1996
[Patent Document 2]
Gazette of Japanese Unexamined Patent Publication No. 14418/1999
[Patent Document 3]
Gazette of Japanese Unexamined Patent Publication No. 10752/1994
[Patent Document 4]
Gazette of Japanese Unexamined Patent Publication No. 13789/2003
[Patent Document 5]
Gazette of Japanese Unexamined Patent Publication No. 94620/1999
[Patent Document 6]
Gazette of Japanese Unexamined Patent Publication No. 821/1987
[Patent Document 7]
Gazette of Japanese Unexamined Patent Publication No. 316145/1999
[Patent Document 8]
Gazette of Japanese Unexamined Patent Publication No. 337382/1999
In general, a relationship between an output of a thermal flow meter using a heat generating resistor and a flow rate of air is expressed by an equation as follows that is known as King's equation.Ih2·Rh=C1+C2√{square root over (Q)})(Th−Ta)  (1)
where, Ih denotes a heating current flowing in the heat generating resistor, Rh denotes a resistance value of the heat generating resistor, Th denotes a surface temperature of the heat generating resistor, Ta denotes an air temperature, Q denotes a flow rate of air, C1, C2 denote a constant defined by the heat generating resistor.
Generally, the output of an air flow meter is set such that the heating current Ih is detected as a voltage value using the detecting resistor. In the engine control unit used for controlling an internal combustion engine, an output current value of the sensor is converted into a flow rate value in reference to the relation of the equation (1) to control a ratio between air and fuel for the internal combustion engine, for example.
A certain linear forming means becomes necessary for detecting a flow rate because a relationship between an output signal of the thermal flow meter and an actual flow rate is a non-linear relation (the square root of the flow rate is a voltage value) as indicated in the equation (1).
In addition, when a dynamic flow variation element such as a pulsation or the like is contained in a flow of fluid, there has been already proposed to provide means as a method for reducing its detection error to be performed such that after a flow rate detecting signal is converted into a digital form, the signal is converted into an air flow rate, changed into a linear form (linear formation), the pulsation amplitude is reduced or pulsation is unified using a low-pass filter and the like in a linear region; and after these operations, the signal is returned back again to a non-linear signal of its original signal form as required and finally the signal is changed into a signal hardly influenced by the pulsation.
In this case, the signal having pulsation amplitude reduced using a low-pass filter and the like shows a response delay, so that the control unit (the engine control unit, for example) requires a processing for recovering its responding characteristic.
At that time, when the low-pass filter is constituted by a digital means, the delay in response of the signal after reduction in pulsation also produces dispersion (variation).
Accordingly, if a sufficient arrangement is not applied to the dispersion in the delay of the response, an effect attained through a recovery processing is also widely dispersed even if the recovery processing for the response characteristic is carried out at the engine control unit side. Further, although the dispersion in response characteristic after the recovering operation can be reduced if a high precision clock signal such as that found in a crystal or the like is used at the digital means, the cost of the sensor device is increased as a result.